Optical spectral filters provide a frequency dependent optical response, such as reflectance or transmittance. Glass filters and thin film interference filters are two well-known approaches for providing optical spectral filters, but the design flexibility provided by these approaches is limited. In particular, design of such filters having arbitrary amplitude and phase response is typically difficult, if not impossible. An alternative approach that provides such design flexibility is to use a dispersive element to separate optical frequencies, followed by one or more filtering elements acting on the individual dispersed frequencies to alter amplitude and/or phase, and a combining element to recombine the filtered frequencies. However, this approach is complex and expensive to implement.
Another approach for optical spectral filtering is the use of a single diffractive surface having adjustable surface elements, e.g. as considered in U.S. Pat. No. 5,905,571 to Butler et al. However, in this reference the only method disclosed for positioning elements of the diffractive surface to provide a desired spectral response is an iterative numerical method, and it is therefore time consuming to change the desired spectral response during operation. Furthermore, the disclosed method does not allow independent specification of desired amplitude and phase responses.
Optical filter design is also considered in U.S. patent application 2004/0145810 to Ranalli. In this work, a Fourier transform of the desired spectral response is computed to derive an impulse response, which is then implemented by adjusting the width and depth of grating lines to set the impulse response amplitude and phase respectively. This work does not address how to make such filters tunable or electrically controllable. It also requires each grating line width and depth to be set individually and precisely to the required values, which tends to complicate fabrication.
Accordingly, it would be an advance in the art to provide a diffractive surface optical spectral filter having surface elements positioned according to a deterministic algorithm for providing a desired amplitude and phase response. A further advance in the art would be such a filter having electrical controllability or tunability, and having simple fabrication.